Electron discharge tube amplifiers



Dec. 25, 1956 G. N. HOUNSFIELD 2,775,655

ELECTRON DISCHARGE TUBE AMPLIFIERS Filed Sept. 17, 1951 lnvenTBr GODFREY NEWBOLD HOUNSFIELD ziffar/ze y United States Patent ELECTRON DISCHARGE TUBE AMPLIFIERS Godfrey Newbold Hounsficid, Iekenham, England, as-

signor to Electric & Musical Industries Limited, Hayes, Engiand, a company of Great Britain Application September 17, 1951, Serial No. 2463M (Ilaims priority, application Great Britain September 2'7, 195% 2 Claims. 01. 179-171 This invention relates to electron discharge tube amplifiers. The amplification of signals by means of two or more electron discharge tubes coupled in cascade is wellknown, and it is known to reduce inaccuracies in the amplified signals due to the characteristics of the tubes by the employment of negative feedback from a later tube to a preceding tube. In a preferred arrangement the feedback path is provided between the anode of the later tube and the cathode of a preceding tube via a transformer provided with tappings which enable the appropriate degree of feedback to be readily provided. As the gain varies with the degree of feedback, the gain of the amplifier can thus be readily adjusted by adjusting the transformer tapping. However, such an arrangement is not suitable where the gain is to be of relatively low value, for example less than unity as is sometimes necessary in computing circuits, since in this case the tapping has to be adjusted to provide a transformer ratio such that the cathode circuit of said preceding tube presents such an impedance that the later tube is prevented from functioning with appreciable gain.

The object of the invention is to provide an improved amplifier in which the gain of the amplifier may be adjusted within relatively widely spaced limits, for example between a gain of and a gain of /2 According to the invention, there is provided an electron discharge tube amplifier comprising a first amplifier tube having at least a cathode, a control electrode and an output electrode, means to apply signals between said control electrode and cathode, means connecting said output electrode to a source of potential, a second amplifier tube having at least a cathode, a control electrode and an output electrode, means coupling the output electrode of said first tube to the control electrode of said second type to provide a cascade connection of said tubes, means coupling the output electrode of said second tube to a source of potential, a negative feedback connection coupling the output electrode of said second tube to the cathode of said first tube, a coupling between the output electrode of said first tube to the cathode thereof to provide separate feedback to increase the cathode impedance of said first tube, thereby to obtain increased negative feedback, and means for deriving output signals from the output electrode of said second tube.

in order that the said invention may be clearly understood and readily carried into effect, it will now be more fully described with reference to the accompanying drawings, which illustrate the invention as applied, by way of example, to a low frequency amplifier and in which:

Figure 1 illustrates a circuit arrangement according to one embodiment of the invention, and

Figure 2 illustrates a further circuit according to the invention.

As shown in Figure 1 of the drawings, the amplifier comprises discharge tubes 1 and 2 each provided with at least a cathode, a control electrode and an anode. The anode of tube 1 is connected via an adjustable tapping 3 to a winding 4 of a transformer, one end of the Wind- 2,775,656 Patented Dec. 25, 1956 ing 4 being connected via a load resistor 5 to a positive H. T. terminal. The junction of said winding 4 and load resistor 5 is connected via a blocking condenser 6 to the control electrode of tube 2, said control electrode being connected via a resistor 7 to a source of negative bias voltage. The cathode of tube 2 is connected to ground and the anode of tube 2 is connected via load resistor 8 to the said positive H. T. terminal, and via a blocking condenser 9 to a tapping on another winding 10 of said transformer, one end of said winding10 being connected to ground. The cathode of said tube 1 is connected to an adjustable tapping 11 on said winding 10 and the control electrode of said first tube is connected via a resistor 12 to a source of negative bias voltage.

In operation, input signals are applied between ground and the control electrode of tube 1, and output signals are derived from the anode of the tube 2 via said winding 10 or a part thereof, or if preferred, the transformer may be provided with a tertiary winding 13 from which said output signals are derived. Negative feedback is applied from the anode of tube 2 to the cathode of tube 1 via the auto-transformer formed by the transformer winding 10 and serves to reduce the distortion in the output signals, and by adjusting the tapping 11, the gain of the circuit is varied in proportion to the tapping ratio. When a very low gain is required, for example of the order of M2, the said tapping requires to be adjusted to a position, for example to the ungrounded end of the winding 10, such that the impedance presented to the anode of said tube 2 would normally be too low for effective feedback. The winding 4 is arranged, as regards ampere turns and direction of winding, to apply positive feedback from the anode of tube 1 to the winding 10 which has the efiect of increasing the impedance presented by the cathode circuit of tube 1 to the anode of tube 2. Said impedance can then be rendered effectively infinite, whereby increased negative feedback from said tube 2 is obtained. Further, the magnetic field of said winding 4 opposes the field induced in the core of the transformer by the said part of said main winding and substantially neutralises the tendency for said core to become saturated.

If desired, instead of the auto-transformer arrangement described, a transformer having separate primary and secondary windings may be employed, but the employment of an auto-transformer improves the frequency response of said first tube.

In a practical case of the above-described arrangement, the tubes 1 and 2 had individual gains of 44 and 16 respectively. With the overall gain adjusted to 1:1, the gain around the feedback loop was 700, the output impedance of the amplifier being 5 ohms. With the gain adjusted to 1:5, the gain round the feedback loop was and the output impedance 25 ohms.

There may be some difficulty in maintaining the H. F. response at the anode of tube 1 due to H. F. coupling between the anodes of the tubes via the transformer windings, and due to leakage inductance and the stray capacity of said transformer winding 4, and also due to stray capacity to earth. In order to overcome these difficulties the anode circuit of said tube 1 may be modified as shown in Figure 2. It will be seen from this figure that the anode of tube 1 is connected to the tapping 3 on winding 4 via a resistor 14, the junction of said resistor 14 and said tapping 3 being connected to ground via a condenser C1. The anode of tube 1 is connected to said blocking condenser 6 via a condenser C2, and the junction of said winding 4 and the anode load resistor 5 of tube 1 is connected via a resistor 16 and condenser 6 to the control electrode of tube 2. Said winding 4 is provided with a grounded screen. It can be shown with this arrangement that if with C1 and C2 removed, R1 is the output impedance at the junction of said tapping 3 and the resistor 14 and R2 the output impedance at the control electrode of tube 2, then if CiRi is substantially equal to CzRz the circuit maintains a good frequency response at the said control electrode without phase shift and isolates said auxiliary Winding from the H. F. portions of the circuit. C1 is to be regarded as including the capacity between said auxiliary winding and the screen.

In circuits as described the gain can be adjusted with a high degree of accuracy and is not substantially afiected by change of tube characteristics. The circuits have a low output impedance and there is little distortion, if any, of the amplified waveform.

Furthermore, a large input signal, even in excess of the H. T. voltage, can be applied directly to the control electrode of the tube 1 and provided the gain of the circuit is adjusted so that the tube 2 receives a difference signal which does not overload it the whole circuit will not be overloaded.

What I claim is:

1. An electron discharge tube amplifier comprising a first amplifier tube having at least a cathode, a control electrode and an output electrode, a transformer having a first winding with one end connected to a point of substantially low fixed potential, a connection between the cathode of said first tube and a first intermediate point on said first Winding, means for applying signals between said control electrode and said low fixed potential point, a high tension source of potential which is positive with respect to said low fixed potential point, a load impedance connecting the output electrode of said tube to said source, a second winding coupled to said transformer and having a point thereon connected to said output electrode to apply positive feedback to said cathode thereby to increase the cathode impedance of said tube, a second amplifier tube having at least a cathode, control electrode, and an output electrode, means coupling the output electrode of said first tube to the control electrode of said second tube to feed signals thereto, an impedance connecting the output electrode of said second tube to said high tension source of potential which is positive with respect to the potential of said low fixed potential point, a connection from the cathode of said second tube to a point of substantially low fixed potential, a negative feedback connection coupling the output electrode of said second tube to a second intermediate point on said first winding, and means for deriving output signals from the output electrode of said second tube.

2. An electron discharge tube amplifier comprising a first amplifier tube having at least a cathode, a control electrode and an output electrode, a transformer having a first winding with one end connected to a point of substantially low fixed potential, a connection between the cathode of said first tube and a first intermediate point on said first winding, means for applying signals between said control electrode and said low fixed potential point, a second Winding coupled to said first winding, a first resistor connecting the output electrode of said tube to a point on said second winding, a high tension source of p0- tential which is positive with respect to said low fixed potential point, a second resistor connecting one end of said second winding to said source, a capacitor connecting the end of said first resistor which is connected to said point on said second winding to a point of substantially low fixed potential, said second winding thereby applying positive feedback to said cathode to increase the cathode impedance of said tube, a second amplifier tube having at least a cathode, control electrode and an output electrode, means coupling the output electrode of said first tube to the control electrode of said second tube to feed signals thereto, said means including a further resistor and a blocking capacitor connected in series, one end of said further resistor being connected to said end of said second winding and the other end of said further resistor being connected through a capacitor to the end of said first resistor connected to said first tube output electrode, an impedance connecting the output electrode of said second tube to said high tension source, a connection from the cathode of said second tube to said low fixed potential point, a negative feedback connection coupling the output electrode of said second tube to a second intermediate point on said first winding, and means for driving output signals from the output electrode of said second tube.

References Cited in the file of this patent UNITED STATES PATENTS 2,323,634 Van Slooter et al. July 6, 1943 2,524,821 Montgomery Oct. 10, 1950 2,545,788 McIntosh Mar. 20, 1951 2,582,603 Phelan Jan. 15, 1952 2,634,335 StolarOif Apr. 7, 1953 2,657,282 Winkel Oct. 27, 1953 FOREIGN PATENTS 526,173 Great Britain Sept. 12, 1940 738,775 Germany Sept. 11, 1943 645,955 Great Britain Nov. 15, 1950 

